It is known in the art that dialkyldithiophosphinic acids form metal complexes with some metals, e.g. zinc, cadmium, cobalt, nickel, copper and the like. This property has resulted in the use of these phosphinic acids as floatation agents and extraction agents.
For example, U.S. Pat. No. 3,966,569 (Reinhardt et al, Issued June 26, 1976) discloses the separation of iron, nickel, zinc, chromium and copper from a solution by three liquid extractions with different organic solutions. In the first extraction copper is removed, zinc and iron in the second, and nickel in the third. The solution remaining is then neutralized to induce precipitation of chromium. After the copper is removed, iron and zinc are removed utilizing an organophosphorous compounds, e.g., a dialkylphosphinic acid. After the removal of iron and zinc, nickel is removed utilizing a dialkyl dithiophosphoric acid. Among the general formulas disclosed for the dialkyl dithiophosphoric acid there is included a dialkyl dithiophosphinic acid.
U.S. Pat. No. 4,348,367 (Rickelton et al, issued Sept. 7, 1982) relates to a process for the extraction of cobalt (II) from an aqueous cobalt (II) bearing solution. In the process an organic-soluble phosphinic acid is utilized. U.S. Pat. No. 4,382,016 (Rickelton et al, issued May 3, 1983) discloses a composition for use in extraction comprising an inert material and an organophosphinic acid. U.S. Pat. No. 4,374,780 (Robertson, issued February 22, 1983) discloses 2,4,4-trimethylpentylphosphinic acid and its use as a selective extractant for cobalt (II) in aqueous cobalt (II)-bearing solutions containing nickel (II).
It is also known that phosphinic acids selectively extract zinc from calcium at a pH of 3 with the continued addition of base to maintain the pH.
None of the foregoing discloses the selective extraction of metals such as zinc, silver, cadmium, copper and the like, against metals such as calcium, magnesium and the like in a low pH, e.g. 0.2-3.0, aqueous solution without having to add additional base to maintain the pH. In many processes aqueous solutions are utilized. These solutions are derived from, e.g., water which can contain such metals as calcium, and magnesium. It is usually desirable to recover certain metals from these aqueous solutions which become deposited therein during a particular process. Thus, for example, in the rayon process zinc is used to control the coagulation of the rayon fibers during the spinning operation. The efficient recovery of zinc used in this process from aqueous solutions containing, for example, calcium, would be highly desirable.
Therefore, a welcome contribution to the art would be a method for the selective extraction of zinc, silver, cadmium, copper, nickel, cobalt and the like from aqueous solutions containing these metals as well as calcium, magnesium and the like. Such a method is provided by this invention.